1. Field of the Invention
The present invention relates to a current control system, and particularly to a control system with operation of selecting resistors.
2. Description of the Prior Art
An electric motor uses current power to provide assist torque and drive machines. The motor is generally connected to the machine with a shaft. In utilization, power is applied to revolve the shaft and the shaft then drives the machine. Because of inertia, the higher the rotation frequency is the less torque is required to rotate the shaft, so the motor requires less current to provide torque. The qualitative relationships between these three variables are shown in FIG. 1A and FIG. 1B. It is obvious that the machine needs different torque in different working regions, so the current power used by the motor also is varied in different working regions. If the motor does not share its power source with other devices, the varied requirement of current only affects the working condition of the power source. However, if the motor shares power source with other equipment, the varied power requirement will affect other equipment, changing the power quantity they can use. An obvious example is that the constant current source is shared by an electrical motor and other machines. When the variation of consumed current of the motor is too large, the others may lack current and loss their ability to work stably and efficiently.
In contemporary electric industry, the system minimization tendency increases the probability that electric motor shares its power source with other equipment and the power source is possibly fixed on a limited voltage or current. For example, the universal series bus (USB) is a new interface for the computer and all peripherals connected through the USB to the computer share a common constant current and fixed voltage source. In these systems, the variation of the consumed current of the motor will significantly affect the stability of the whole system. Therefore, this problem must be solved.